1. Field of the Invention
This invention relates generally to static VAR generators and more particularly to static VAR generators having auxiliary shutdown firing generators for isolation of the static VAR generator from the AC network in the event the VAR generator malfunctions.
2. Description of the Prior Art
In order to ensure shutdown capability, a full inductor current must always be maintained in the thyristor phase controlled inductor of the VAR generator during its shutdown. In the present VAR generator designs, the firing instance of the phase control pulses is derived from the power system's voltage in several stages. First, potential transformers supply input signals to a phase-lock type synchronous timing circuit. The synchronous timing circuit provides the necessary information for a thyristor phase angle control circuitry. Finally, the phase control determines the firing instance of the necessary full current phase control firing pulses. The phase control pulses obtained are transmitted from the phase control circuitry to a light emitting diode drive and protection circuitry. When a phase control pulse is received, the LED drive generates a light pulse that turns on the optically coupled firing circuits in the high voltage thyristor array of the phase controlled inductor. In order to obtain a full inductor current successfully, all of the above circuitry must be operational. One of the major concerns of both the supplier and the user of static VAR generators is the assurance of long life and reliable operation of the equipment. In some cases the shut down of the VAR generator, due to an internal failure, may require shutting down a complete system. One example of a shutdown scheme is found in U.S. Pat. No. 4,047,097, entitled "Apparatus And Method For Transient Free Energization And Deenergization Of Static VAR Generators", assigned to the assignee of the present invention.
The VAR generator power circuit basically consists of a fixed capacitor bank and a thyristor controlled inductor bank. When both the capacitor and the inductor banks are connected or switched across the AC voltage, the capacitor and inductor currents cancel each other out, therefore, the power line current supplied to the combined banks becomes practically zero. This zero current then can be interrupted in an easy, transient free manner by the main circuit breaker of the VAR generator during a normal VAR generator shutdown. If, at any time, during a shutdown the thyristor controlled inductor is inoperative, the main circuit breaker would have to interrupt capacitive discharge current with the possibility of the arc restriking in the circuit breaker and causing severe damage to the capacitor due to overvoltages. When the main circuit breaker is opened to deenergize a capacitor bank, the initial voltage across the breaker is zero. A half cycle later when the system voltage has reversed, and the capacitor voltage is still the same as when the breaker started opening, the voltage across the breaker is doubled. If the breaker restrikes on this double voltage, a high oscillatory current flows. The current is high because the voltage is doubled.